Electrostatic discharge protection device having a graded junction and method for forming the same

A method of forming a protection device to protect an integrated circuit from an electrostatic discharge, the method comprising the steps of:

• forming, in a substrate having a first conductivity type, a pad ring structure having an inner pad ring and an outer pad ring, both the inner and outer pad rings having a second conductivity type but having different concentration levels of dopant, the inner pad ring directly abutting the substrate having the first conductivity type, the inner and outer pad rings extending into the substrate to approximately a same selected depth;
• forming, in the substrate, a rail ring structure having an inner rail ring and an outer rail ring, both the inner and outer rail rings formed by the second conductivity type but having different concentration levels of dopant, the inner rail ring directly abutting the substrate having the first conductivity type, the inner and outer rail rings extending into the substrate to the selected depth;
• coupling the inner pad ring to a bonding pad;
• and
• coupling the inner rail ring to a ground rail used by the integrated circuit.

A method of forming a protection device comprising the steps of:

• forming an outer pad ring of a first conductivity at a first concentration in a substrate of a second conductivity to a selected depth;
• forming an inner pad ring of the first conductivity at a second concentration into the substrate within the outer pad ring, the second concentration higher than the first concentration and extending onto the substrate to approximately the selected depth;
• forming a rail contact of the first conductivity at a third concentration in the substrate;
• implanting a buried layer beneath the inner pad ring and implanting a second buried layer beneath the rail contact;
• coupling the inner pad ring to a first portion of the device;
• and
• coupling the rail contact to a second portion of the device.